Heretofore, flexible testers for small batteries employed finite conductive layers having a greater than atomic thickness which were adversely affected by repeated cycles of extreme bending. In addition, these earlier conductive layers were tapered for creating a test current density gradient which produced a test equilibrium temperature gradient along the substrate. A "hot spot" at the nip of the taper where the current density was the highest limited the dynamic range of the conductive layers. These earlier testers did not have a uniform test current density or a uniform test equilibrium temperature.
Long testers with smaller temperature gradients were generally more accurate than shorter testers with higher temperature gradients. The testers employed a single thermochromic indicator mounted along the temperature gradient next to a status scale. The thermochromic had a single response temperature. The user determined the status of the battery by observing the point on the scale where the increasing equilibrium temperature along the temperature gradient was equal to the fixed response temperature of the thermochromic. There was no provision in these prior art devices for varying the length of the conductive layers in order to control the resulting current density.
The placement of the thermochromic indicator along the temperature gradient also affected the accuracy of the testing. Mis-registration between the indicator and the temperature gradient caused the indicator to respond late in the test cycle resulting in an under evaluation of the battery strength.
U.S. Pat. No. 4,006,414 issued on Feb. 1, 1977 to Robert Parker teaches a tapered conductive metallic foil for heating the visual indicator. U.S. Pat. No. 4,702,563 issued on Oct. 27, 1987, and U.S. Pat. No. 4,702,564 issued on Oct. 27, 1987, and U.S. Pat. No. 4,726,661 issued on Feb. 23, 1988, all to Robert Parker; teach a tapered epoxy conductor printed or painted on the substrate of the tester. U.S. Pat. No. 4,835,475 issued on May 30, 1989 to Hanakura shows a dry battery tester with a conductive heater and thermochromic layer on a film base.